1. Field of the Invention
The disclosure relates to a system and method for transmitting signals, and more particularly, to a system and method for transmitting signals by using a digital-to-analog converter without using the electrical mixer to generate a reference optical carrier such that the ratio of the power between the optical carrier and the signal can be adjusted individually to optimize power efficiency.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Current wideband transmission technology is improved very quickly. The transmission speed of the very-high-bit-rate Digital Subscriber Line 2 (VDSL2) has reached 100 Mbps, and transmission speeds of the Wireless LAN (WLAN) and the Ultra-Wide Band (UWB) systems have reached 100 Mbps and will rise to 1 Gbps in the near future. Transmission speed is important because electronic devices such as CCTV and medical sensors transmit data via the network, in addition to the PC and cell phone. The optical fiber network possesses advantages such as high stability and ultra wideband and long-distance transmission ability; therefore, it is very suitable for connecting wired or wireless wideband networks to a local terminal. However, the current available architecture of the optical fiber network, Fiber-to-the-x (FTTx) has a bandwidth of only between 1 and 2.5 Gbps, which is not wide enough to transmit the large amounts of data required in the near future.
The conventional transmission system uses the Orthogonal Frequency Division Multiplexing (OFDM) technique to carry signals of 10 Gbps on 2 GHz bandwidth. The 10 Gbps optical transceiver for the FTTx can be implemented by using the multilevel optical modulation technique incorporating the OFDM multi-carrier technique such that a 10 Gbps transmission speed can be achieved by using the low speed laser and PIN. Using the OFDM multi-carrier technique and the Quadrature Amplitude Modulation (QAM) technique to convert the high-speed 10 Gbps series data into multiple parallel channel data carried in a bandwidth between 0.1 and 2 GHz not only decreases the data transmission rate and the occupied bandwidth of each subcarrier to solve the fiber dispersion problem, but also avoids the feedback equalization issue due to using a single carrier. In addition, the OFDM system needs a multiplier for each subcarrier channel, which enables individual gain-adjusting of each subcarrier channel to allow a uniform channel response. Consequently, this system can be applied to the 10 Gbps high-speed circuit.
However, the conventional optical OFDM produces the Inter-Modulation Distortion (IMD) problem at the receiving end. The receiver used in the conventional optical OFDM to convert optical signals into electrical signals has the square effect, which squares the received OFDM signals and generates the subcarrier beat phenomena. The frequency of the beat occurs at the frequency difference between each two of the subscarriers, from baseband to the bandwidth originally occupied by the signals; these beat signals are known as IMD. Since the band of the IMD and that of the OFDM signal overlap, very serious interference and signal errors occur.
The IMD interference could be solved by up-conversing the OFDM signals to a frequency higher than the original bandwidth, which is the band where the IMD occurs. This solution can be explained by the following equation:
            [                                    m            ⁡                          (              t              )                                ⁢                      cos            ⁡                          (                                                ω                  C                                +                                  ω                  RF                                            )                                ⁢          t                +                  cos          ⁢                                          ⁢                      ω            C                    ⁢          t                    ]        2    =                                          m            2                    ⁡                      (            t            )                          ⁢                              cos            2                    ⁡                      (                                          ω                C                            +                              ω                RF                                      )                          ⁢        t            +                        cos          2                ⁢                  ω          C                ⁢        t            +              2        ⁢                                  ⁢                  m          ⁡                      (            t            )                          ⁢        cos        ⁢                                  ⁢                  ω          C                ⁢        t        ×                  cos          ⁡                      (                                          ω                C                            +                              ω                RF                                      )                          ⁢        t              ≈                            m          2                ⁡                  (          t          )                    +                        m          ⁡                      (            t            )                          ⁢        cos        ⁢                                  ⁢                  ω          RF                ⁢        t            +              high        ⁢                                  ⁢        frequency        ⁢                                  ⁢        terms            m(t)cos(ωC+ωRF)t+cos ωCt in the square bracket represents the optically modulated signals at the transmitting end, m(t) represents the signals to be received, w represents the frequency of the optical carrier, and ωRF represent the frequency after up-conversion at the transmitting end. Since the signals are up-converted to ωRF at the transmitting end, the frequency of the signals is higher than that of the optical carrier by ωRF after the optical modulator. At the receiving end, the signals and the optical carrier after the square effect of the receiver generate the square terms from itself and the square terms from one another. The high frequency terms can be filtered, but 2m(t) and m(t)cos ωRF t (the signal to be received) are retained. Since the m(t) has been up-converted to the frequency of ωRF, which is higher than the band where the IMD occurs, and the IMD interference can be solved.
In summary, the conventional technique uses the electrical mixer to conduct the up-conversion so as to solve the IMD interference; however, the use of the electrical mixer decreases the signal to noise ratio. Another conventional technique for solving the IMD interference uses a portion (high-frequency band) of the digital-to-analog converter (DAC) to transmit the signals, but this technique uses half the bandwidth of the DAC.